1. Field of the Invention
The present invention relates to an anodizing apparatus for anodizing a conducting film formed on a substrate used in a thin-film-transistor-operated (TFT-operated) active-matrix liquid crystal display device and the like.
2. Description of the Related Art
A TFT panel used in a TFT-operated active-matrix liquid crystal display device is constructed in the manner shown in FIGS. 1A and 1B, for example.
Referring to FIG. 1A, a gate line GL, for use as an address line, and a drain line DL, for use as a data line, are formed crossing each other on a transparent glass substrate SG, with a gate insulating film GI (mentioned later) and a crossing insulating film II between them. In the region near this crossing section, a thin film transistor FT is formed such that its gate G and drain D are connected to the gate line GL and the drain line DL, respectively. A source S of the transistor FT is connected to a pixel electrode P.
Referring to FIG. 1B, the gate insulating film GI is put on the transparent glass substrate SG so as to cover the gate line GL and the gate G. A semiconductor film SC, formed of amorphous silicon, the drain line DL, and the pixel electrode p are stacked in a predetermined pattern on the gate insulating film GI. The drain D and the source S are formed individually over the semiconductor film SC with ohmic contact layers O between the stacked layers. A blocking layer B is provided on the semiconductor film SC and interposed between the drain D and the source S. A protective film PF is formed over the whole top area of the resulting structure except a predetermined region of the pixel electrode P.
According to the TFT panel constructed in this manner, if the gate insulating film GI, which isolates the gate line GL and the gate G, constituting a lower conducting film, from the drain line DL, drain D, etc., constituting an upper conducting film, is subject to pinholes, cracks, or other defects, the lower and upper conducting films will inevitably be shorted at those defective portions.
In the TFT panel described above, therefore, the gate line GL and the gate G, which constitute the lower conducting film, are anodized except terminal portions of the gate line GL so that an oxide film is formed on the surface of the lower conducting film. This oxide film and the gate insulating film GI doubly isolate the lower and upper conducting films from each other.
The lower conducting film is anodized by dipping the substrate, having the conducting film thereon, in an electrolyte so that the conducting film faces a cathode, and then applying voltage between the conducting film, for use as an anode, and the cathode. When the voltage is thus applied between the conducting film and the cathode in the electrolyte, the conducting film as the anode undergoes a formation reaction such that it is anodized gradually from its surface, thereby forming the oxide film on its surface. In this anodization, a resist mask is used to cover unoxidized portions (terminal portions of the gate line) of the conducting film which should be prevented from being oxidized.
Conventionally, the anodization of the conducting film on the substrate is conducted by means of a batch-processing anodizing apparatus which collectively anodizes the respective conducting films of a plurality of substrates (e.g., about ten in number).
In general, the anodizing apparatus comprises an electrolyte tank, washing tank, drying chamber, substrate supporting frame, and supporting frame transportation mechanism. The electrolyte tank is filled with an electrolyte, in which cathodes as many as the substrates to be batch-processed are arranged at intervals. The washing tank is used to wash the substrates whose conducting films are anodized in the electrolyte tank. The drying chamber is used to dry the washed substrates. The substrate supporting frame supports a predetermined number of substrates to be batch-processed so that the substrates are arranged at intervals corresponding to the intervals between the cathodes in the electrolyte tank.
In the above-described conventional anodizing apparatus which collectively anodizes the respective conducting films of the substrates, however, the electrolyte tank used is a large-sized tank having a large enough capacity to allow a plurality of substrates to be simultaneously dipped in the electrolyte, and the cathodes as many as the substrates to be batch-processed must be arranged in the electrolyte tank. Thus, the electrolyte tank requires so large a capacity that the equipment cost of the apparatus and, therefore, the cost of anodization of the conducting film on each substrate inevitably increase.
With use of the batch-processing anodizing apparatus, attaching to or detaching e.g. about ten substrates to be batch-processed from the supporting frame takes much time, and it is difficult to process the ten substrates uniformly in conducting pre- and post-treatments for anodization together. Thus, the processing time for each substrate is long, and the cost of anodization is high.
Meanwhile, the thickness of the oxide film formed on the surface of the conducting film is believed to depend on a formation voltage applied between the conducting film to be oxidized and the cathode. Conventionally, therefore, the conducting film is anodized by controlling the formation voltage between the conducting film and the cathode in the following manner.
FIG. 2 shows a control pattern of the formation voltage used in a conventional anodizing method. Conventionally, the formation voltage applied between the conducting film to be oxidized and the cathode is increased to a predetermined value with the value of a formation current flowing through the conducting film (or current flowing between the conducting film and the cathode via the electrolyte) kept constant. After the predetermined voltage value is attained, application of the formation voltage at this value is continued for a certain period of time. When the application of the voltage is stopped, thereafter, the anodization is finished.
Thus, according to this anodizing method, the formation voltage applied between the conducting film to be oxidized and the cathode is increased to the predetermined value in a constant-current mode, and the voltage at this value is then applied in a constant-voltage mode for the given period of time. Conventionally, the application of the formation voltage in the constant-voltage mode is continued until the value of the current flowing through the conducting film to be oxidized is lowered to a preset value Va (approximately zero) or below. When the current value is lowered to the preset value Va or below, it is concluded that the oxide film has a desired thickness, whereupon the anodization is finished.
FIG. 3 is a sectional view of a conducting film 2' (e.g., gate line formed on a substrate 1') anodized by the anodizing method described above. An oxide film 2a' formed on the surface of the conducting film 2' has a dielectric strength substantially equivalent to the formation voltage, between an unoxidized portion of the conducting film 2' and another conducting film (not shown) formed on the oxide film 2a'.
As shown in FIG. 3, however, the oxide film 2a' formed on the surface of the conducting film by the aforementioned conventional anodizing method involves defective portions a. When the voltage is applied between the unoxidized portion of the conducting film and the other conducting film formed on the oxide film, therefore, the oxide film inevitably undergoes dielectric breakdown in the vicinity of the defective portions a.
In the case where the conducting film to be oxidized is an aluminum alloy film, the formation voltage applied between the conducting film and the cathode is conventionally increased to a value such that an oxide film with a suitable thickness is formed with the formation current flowing through the conducting film kept constant so that the current density is 2.5 mA/cm.sup.2 or below (1.5 mA/cm.sup.2 in FIG. 4).
The oxide film (Al.sub.2 O.sub.3), thus formed on the surface of the aluminum alloy film in this condition, is a microcrystalline barrier film which enjoys a high genuine dielectric breakdown strength (nondefective-state dielectric breakdown strength).
Although the oxide film (Al.sub.2 O.sub.3) formed on the surface of the aluminum alloy film by the conventional method has a high genuine dielectric breakdown strength, however, it involves many local low-strength portions since it is a microcrystalline barrier film containing fine crystalline particles. Thus, dielectric breakdown can be caused by an electric field of a relatively low intensity, e.g., about 3 MV/cm.